This invention relates generally to methods and apparatus for desorption and ionization of analytes for the purpose of subsequent scientific analysis by such methods, for example, as mass spectrometry (MS) or biosensors. Generally, analysis by mass spectrometry involves the vaporization and ionization of a small sample of material, using a high energy source, such as a laser, including a laser beam. The material is vaporized from the surface of a probe tip into the gas or vapor phase by the laser beam, and, in the process, some of the individual molecules are ionized by the gain of a proton. The positively charged ionized molecules are then accelerated through a short high voltage field and let fly (drift) into a high vacuum chamber, at the far end of which they strike a sensitive detector surface. Since the time-of-flight is a function of the mass of the ionized molecule, the elapsed time between ionization and impact can be used to determine the molecule's mass which, in turn, can be used to identify the presence or absence of known molecules of specific mass.
All known prior art procedures which present proteins or other large biomolecules on a probe tip for laser desorption/ionization time-of-flight mass spectrometry (TOF) rely on the preparation of a crystalline solid mixture of the protein or other analyte molecule in a large molar excess of acidic matrix material deposited on the bare surface of a metallic probe tip. (The sample probe tip typically is metallic, either stainless steel, nickel plated material or platinum). Embedding the analyte in such a matrix was thought to be necessary in order to prevent the destruction of analyte molecules by the laser beam. The laser beam strikes the solid mixture on the probe tip and its energy is used to vaporize a small portion of the matrix material along with some of the embedded analyte molecules. Without the matrix, the analyte molecules are easily fragmented by the laser energy, so that the mass, and identity, of the original macromolecule is very difficult or impossible to determine.
This prior art procedure has several limitations which have prevented its adaptation to automated protein or other macrobiological molecular analysis. First, in a very crude sample it is necessary to partially fractionate (or otherwise purify the sample as much as possible) to eliminate the presence of excessive extraneous materials in the matrix/analyte crystalline or solid mixture. The presence of large quantities of components may depress the ion signal (either desorption, ionization and/or detection) of the targeted analyte. Such purification is time-consuming, expensive, typically results in low recovery (or complete loss) of the analyte, and would be very difficult to do in an automated analyzer.
Second, while the amount of analyte material needed for analysis by the prior art method is not large (typically in a picomole range), in some circumstances, such as tests on pediatric patients, analyte fluids are available only in extremely small volumes (microliters) and may be needed for performing several different analyses. Therefore, even the small amount (i.e., volume) needed for preparation of the analyte/matrix crystalline mixture for a single analysis may be significant. Also, only a tiny fraction (a few thousandths or less) of analyte used in preparing the solid analyte/matrix mixture for use on the probe tip is actually consumed in the desorption or mass spectrometric analysis. Any improvement in the prior art procedure which would make it possible to 1) use much less analyte, 2) to locate the analyte or multiple analytes on the probe tip or surface in a predetermined location, 3) to perform repeated analyses of the same aliquot of analyte (e.g., before and after one or more chemical and or enzymatic reactions), and 4) to conduct the test in a more quantitative manner, would be highly advantageous in many clinical areas.
Third, the analyte protein, or other macromolecule, used in preparing the solid solution of analyte/matrix for use on the probe tip is not suitable for any subsequent chemical tests or procedures because it is bound up (i.e., embedded) in the matrix material. Also, all of the matrix material used to date is strongly acidic, so that it would adversely affect many chemical reactions which might be attempted on the mixture in order to modify the analyte molecules for subsequent examination. Any improvement in the procedure which made it possible to conduct subsequent chemical modifications or reactions on the analyte molecules, without removing them from the matrix or the probe tip or without “matrix” altogether, would be of enormous benefit to researchers and clinicians.
The first successful molecular mass measurements of intact peptides and small proteins (only up to about 15 kDa) by any form of mass spectrometry were made by bombarding surfaces with high energy particles (plasma desorption and fast atom bombardment mass spectrometry); this breakthrough came in 1981 and 1982. Improvements came in 1985 and 1986, however, yield (signal intensities), sensitivity, precision, and mass accuracy remained relatively low. Higher molecular mass proteins (about 20 to 25 kDa) were not observed except on rare occasions; proteins representing average molecular weights (approximately 70 kDa) were not ever observed with these methods. Thus, evaluation of most proteins by mass spectrometry remains unrealized.
In 1988, Hillenkamp and his coworkers used UV laser desorption time-of-flight mass spectrometry and discovered that when proteins of relatively high molecular mass were deposited on the probe tip in the presence of a very large molar excess of an acidic, UV absorbing chemical matrix (nicotinic acid) they could be desorbed in the intact state. This new technique is called matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Note that laser desorption time-of-flight mass spectrometry (without the chemical matrix) had been around for some time, however, there was little or no success determining the molecular weights of large intact biopolymers such as proteins and nucleic acids because they were fragmented (destroyed) upon desorption. Thus, prior to the introduction of a chemical matrix, laser desorption mass spectrometry was essentially useless for the detection of specific changes in the mass of intact macromolecules. Note that the random formation of matrix crystals and the random inclusion of analyte molecules in the solid solution is prior art.
There are a number of problems and limitations with the prior art methods. For example, previously, it has been found that it is difficult to wash away contaminants present in analyte or matrix. Other problems include formation of analyte-salt ion adducts, less than optimum solubility of analyte in matrix, unknown location and concentration of analyte molecules within the solid matrix, signal (molecular ion) suppression “poisoning” due to simultaneous presence of multiple components, and selective analyte desorption/ionization. Prior investigators, including Karas and Hillenkamp have reported a variety of techniques for analyte detection using mass spectroscopy, but these techniques suffered because of inherent limitations in sensitivity and selectivity of the techniques, specifically including limitations in detection of analytes in low volume, undifferentiated samples. (Hillenkamp, Bordeaux Mass Spectrometry Conference Report, pp. 354–62 (1988); Karas and Hillenkamp, Bordeaux Mass Spectrometry Conference Report, pp. 416–17 (1988); Karas and Hillenkamp, Analytical Chemistry, 60:2299 (1988); Karas, et al., Biomed. Environ. Mass Spectrum (in press).) The use of laser beams in time-of-flight mass spectrometers is shown, for example, in U.S. Pat. Nos. 4,694,167; 4,686,366, 4,295,046, and 5,045,694, incorporated by reference.
The successful volatilization of high molecular weight biopolymers, without fragmentation, has enabled a wide variety of biological macromolecules to be analyzed by mass spectrometry. More importantly perhaps, it has illustrated the potential of using mass spectrometry more creatively to solve problems routinely encountered in biological research. Most recent attention has been focused on the utility of matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS), largely because it is rapid (min), sensitive (<pmol sample required), and permits complex mixtures to be analyzed.
Although MALDI-TOF MS continues to be useful for the static determination/verification of mass for individual analytes, in the case of biopolymers, it is often differences in mass that provide the most important information about unknown structures. Thus, for routine use in structural biology, an unfortunate limitation of the MALDI-TOF MS technique relates to sample preparation and presentation (deposition) on an inert probe element surface, specifically, the requirement that analytes be embedded (i.e., co-solidified) on the probe surface in a freshly prepared matrix of crystalline organic acid. The random distribution of analyte in a heterogeneous display of crystal matrix on the probe element surface requires the deposition of far more analyte or sample than is needed for the laser desorption process, even for the collection of more than adequate mass spectra (e.g., multiple sets of 100 shots each). The remaining portion of the analyte is usually not recovered for additional analyses or subsequent characterizations. Even though 1 to 10 pmol (sometimes less) of analyte are typically required for deposition on the probe surface, it has been estimated that less than a few attomoles are consumed during laser desorption. Thus, only 1 part in 105 or 106 of the applied analyte may be necessary; the rest is lost.
Another important loss of potential data associated with the embedding of analyte in a solid matrix is the reduction or the complete elimination of ability to perform subsequent chemical and/or enzymatic modifications to the embedded analyte (e.g., protein or DNA) remaining on the probe surface. Only another aliquot of analyte, or the ability to recover the embedded analyte free of matrix (difficult with low recovery), allows what we now refer to as differential mass spectrometry to be performed to derive structural data.
In addition, there has been limited application of MS in biological fields, likely due to the fact that many biologists and clinicians are intimidated by MS and/or skeptical in regard to its usefulness. Further, MS is perceived as inaccessible or too costly, particularly because SDS polyacrylamide gel electrophoresis is an adequate substitute in some instances where MALDI would be applied (e.g., separation of crude biological fluids). In addition, MALDI has had little exposure in biological and clinical journals.